Apparatus and method for operating an apparatus for determining and/or monitoring at least one physical process variable

Abstract

A method for operating an apparatus which has an oscillatable unit. The oscillatable unit is excited to oscillate by means of a first frequency sweep within a predetermined frequency band with successive, discrete exciter frequencies of increasing or decreasing frequency. A first exciter frequency is ascertained, in the case of which, during the first frequency sweep, at least one predeterminable criterion is fulfilled. The oscillatable unit is excited by means of a second frequency sweep, wherein the frequency band, compared with the first frequency sweep, is run through in the opposite direction. A second exciter frequency is ascertained, in the case of which, during the second frequency sweep, the at least one predeterminable criterion is fulfilled. From the first exciter frequency and the second exciter frequency, via formation of an average, a measuring frequency for determining and/or monitoring at least one process variable is determined.

The invention claimed is: 
     
       1. A method for operating an apparatus for determining and/or monitoring at least one physical process variable of a medium, which apparatus has an oscillatable unit, comprising the steps of:
 oscillating the oscillatable unit to oscillate by means of a first frequency sweep within a predetermined frequency band in the working range of the oscillatable unit using transmission signals with successive, discrete exciter frequencies of increasing or decreasing frequency; 
 receiving corresponding oscillations of the oscillatable unit in the form of received signals; 
 ascertaining a first exciter frequency, in the case of which, during the first frequency sweep, at least one predeterminable criterion is fulfilled; 
 oscillating the oscillatable unit to oscillate by means of a second frequency sweep within the predetermined frequency band in the form of transmission signals with the same successive, discrete exciter frequencies; 
 running through the frequency band, compared with the first frequency sweep, in opposite direction; 
 receiving the corresponding oscillations of the oscillatable unit in form of said received signals; 
 ascertaining a second exciter frequency, in the case of which, during the second frequency sweep, the at least one predeterminable criterion is fulfilled; and 
 determining a measuring frequency for determining and/or monitoring at least one process variable, via formation of an average from said first exciter frequency and said second exciter frequency. 
 
     
     
       2. The method as claimed in  claim 1 , wherein:
 ascertained as the average measuring frequency is the resonance frequency or a frequency, in the case of which a predeterminable phase shift between transmission signal and received signal is present, especially an eigenfrequency of the oscillatable unit. 
 
     
     
       3. The method as claimed in  claim 1 , wherein:
 predetermined as criterion for determining the first exciter frequency and the second exciter frequency is the presence of a maximum or minimum amplitude and/or a particular phase shift between the transmission signal and the received signal. 
 
     
     
       4. The method as claimed in  claim 1 , wherein:
 after determining the average measuring frequency, the oscillatable unit is excited to oscillate with the measuring frequency for determining and/or monitoring at least one process variable. 
 
     
     
       5. The method as claimed in  claim 1 , wherein:
 after determining the average measuring frequency, the average measuring frequency is evaluated as regards at least one process variable, and another set of first and second frequency sweeps for another ascertaining of the average measuring frequency are performed. 
 
     
     
       6. The method as claimed in  claim 1 , wherein:
 as the process variable, at least one of the variables from the group comprising fill level, density, viscosity, mass flow and volume flow is determined and/or monitored. 
 
     
     
       7. The method as claimed in  claim 1 , wherein:
 a difference between the first exciter frequency and the second exciter frequency is ascertained and from the difference between the first exciter frequency and the second exciter frequency, the degree of damping of the oscillations is determined. 
 
     
     
       8. The method as claimed in  claim 7 , wherein:
 based on the degree of damping in the case of oscillation in air, it is ascertained whether accretion is clinging to the oscillatable unit or whether the oscillatable unit is corroded. 
 
     
     
       9. The method as claimed in  claim 7 , wherein:
 from the degree of damping of the oscillations, viscosity of the medium is determined. 
 
     
     
       10. The method as claimed in  claim 9 , wherein:
 progression of the degree of damping is monitored as a function of time, and therefrom, a change in viscosity or accretion or corrosion is detected. 
 
     
     
       11. An apparatus for determining and/or monitoring at least one physical process variable of a medium comprising:
 a mechanically oscillatable unit; 
 a drive/receiving unit, which is arranged to excite said oscillatable unit by means of a first frequency sweep within a predetermined frequency band in the working range of the oscillatable unit using transmission signals with successive, discrete exciter frequencies of increasing or decreasing frequency; 
 an electronics unit for open and/or closed loop control of the oscillation excitement and for evaluating the received signals and determining the process variables; 
 wherein said electronics unit is arranged to receive corresponding oscillations of the oscillatable unit in the form of received signals; 
 wherein said electronics unit is arranged to ascertain a first exciter frequency, in the case of which, during the first frequency sweep, at least one predeterminable criterion is fulfilled; 
 wherein said drive/receiving unit is further arranged to oscillate the oscillatable unit by means of a second frequency sweep within the predetermined frequency band in the form of transmission signals with the same successive, discrete exciter frequencies; 
 wherein said drive/receiving unit is arranged to run through the frequency band, compared with the first frequency sweep, in opposite direction; 
 wherein said electronics unit is arranged to receive the corresponding oscillations of the oscillatable unit in form of said received signals; 
 wherein said electronics unit is arranged to ascertain a second exciter frequency, in the case of which, during the second frequency sweep, the at least one predeterminable criterion is fulfilled; and 
 wherein said electronics unit is embodied to determine a measuring frequency for determining and/or monitoring at least one process variable, via formation of an average from said first exciter frequency and said second exciter frequency.